Functional characterization and Me2+ ion specificity of a Ca2+–citrate transporter from Enterococcus faecalis

Blancato, Víctor Sebastián; Magni, Christian; Lolkema, Juke S.

doi:10.1111/j.1742-4658.2006.05509.x

Cited by 31 publications

(36 citation statements)

References 23 publications

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“…Furthermore, citrate has been shown to be transported in the form of a complex with Ca 2þ , Cd 2þ , Co 2þ , Mg 2þ , Mn 2þ , Ni 2þ , Pb 2þ , Sr 2þ or Zn 2þ , transport rates being related to the specific size of the metal ion. [30] Metals taken up by this pathway can enter cells at a faster rate than metals entering by the cation transporters. In observations made using the green alga Pseudokirchneriella subcapitata, Cd uptake was 2 times faster in the presence of citrate than when similar concentrations of free Cd were buffered by nitrilotriacetic acid (NTA).…”

Section: Introductionmentioning

confidence: 99%

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When are metal complexes bioavailable?

Zhao¹,

Campbell²,

Wilkinson³

2016

Environ. Chem.

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Environmental context. The concentration of a free metal cation has proved to be a useful predictor of metal bioaccumulation and toxicity, as represented by the free ion activity and biotic ligand models. However, under certain circumstances, metal complexes have been shown to contribute to metal bioavailability. In the current mini-review, we summarise the studies where the classic models fail and organise them into categories based on the different uptake pathways and kinetic processes. Our goal is to define the limits within which currently used models such as the biotic ligand model (BLM) can be applied with confidence, and to identify how these models might be expanded.Abstract. Numerous data from studies over the past 30 years have shown that metal uptake and toxicity are often best predicted by the concentrations of free metal cations, which has led to the development of the largely successful free-ion activity model (FIAM) and biotic ligand model (BLM). Nonetheless, some exceptions to these classical models, showing enhanced metal bioavailability in the presence of metal complexes, have also been documented, although it is not yet fully understood to what extent these exceptions can or should be generalised. Only a few studies have specifically measured the bioaccumulation or toxicity of metal complexes while carefully measuring or controlling metal speciation. Fewer still have verified the fundamental assumptions of the classical models, especially when dealing with metal complexes. In the current paper, we have summarised the exceptions to classical models and categorised them into five groups based on the fundamental uptake pathways and kinetic processes. Our aim is to summarise the mechanisms involved in the interaction of metal complexes with organisms and to improve the predictive capability of the classic models when dealing with complexes.

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Section: Introductionmentioning

confidence: 99%

“…This mechanism has been observed for ligands with established biological functions and their own specialised channels or transporters. [27,28] For example, for some bacteria, citrate, which can be used as a carbon source, [29,30] is taken up by citrate transporters (e.g. CitM and CitH).…”

Section: Introductionmentioning

confidence: 99%

When are metal complexes bioavailable?

Zhao¹,

Campbell²,

Wilkinson³

2016

Environ. Chem.

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“…This protein was expressed in Escherichia coli and functionally characterized in membrane vesicles prepared from these cells. Citrate uptake was dependent on the presence of divalent metal ions (Ca 2ϩ , Sr 2ϩ , Mn 2ϩ , Cd 2ϩ , and Pb 2ϩ ) (5).…”

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Transcriptional Regulation of the Citrate Gene Cluster of Enterococcus faecalis Involves the GntR Family Transcriptional Activator CitO

et al. 2008

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The genome of the gram-positive bacterium Enterococcus faecalis contains the genes that encode the citrate lyase complex. This complex splits citrate into oxaloacetate and acetate and is involved in all the known anaerobic bacterial citrate fermentation pathways. Although citrate fermentation in E. faecalis has been investigated before, the regulation and transcriptional pattern of the cit locus has still not been fully explored. To fill this gap, in this paper we demonstrate that the GntR transcriptional regulator CitO is a novel positive regulator involved in the expression of the cit operons. The transcriptional analysis of the cit clusters revealed two divergent operons: citHO, which codes for the transporter (citH) and the regulatory protein (citO), and upstream from it and in the opposite direction the oadHDB-citCDEFX-oadA-citMG operon, which includes the citrate lyase subunits (citD, citE, and citF), the soluble oxaloacetate decarboxylase (citM), and also the genes encoding a putative oxaloacetate decarboxylase complex (oadB, oadA, oadD and oadH). This analysis also showed that both operons are specifically activated by the addition of citrate to the medium. In order to study the functional role of CitO, a mutant strain with an interrupted citO gene was constructed, causing a total loss of the ability to degrade citrate. Reintroduction of a functional copy of citO to the citO-deficient strain restored the response to citrate and the Cit ؉ phenotype. Furthermore, we present evidence that CitO binds to the cis-acting sequences O 1 and O 2 , located in the cit intergenic region, increasing its affinity for these binding sites when citrate is present and allowing the induction of both cit promoters.Enterococcus faecalis, a gram-positive catalase-negative coccus, is a natural member of the human and animal microflora. This ubiquitous microorganism has numerous fields of interest due to its importance as a cause of nosocomial infections and to its utilization in the food industry. With respect to the latter, the bacteria are involved in the ripening process and in aroma development of diverse cheeses. These positive effects have been attributed to specific biochemical traits such as lipolytic activity and citrate utilization. Citrate metabolism has been extensively studied in bacteria, both from a basic and applied point of view, since citrate fermentation plays an important role during the production of diverse kinds of drinks and foods (1-3, 7, 8, 12, 15, 16, 21, 28, 29, 31-33, 48-51).Citrate fermentation was recently investigated in E. faecalis, and the contribution to aroma development was characterized (18,46,47). In this microorganism, the first steps of the citrate degradative pathway are shared with other citrate-fermenting bacteria. Initially, citrate is converted to oxaloacetate and acetate by the enzyme citrate lyase. Then, oxaloacetate is decarboxylated to pyruvate. This compound is subsequently degraded to acetate, CO 2 , and formate with the generation of ATP by the acetate kinase enzyme. In addit...

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“…It is believed that these organisms take up complexed citrate, because it is predominantly available as such in their environment, or they allow access to critical metal ions, such as iron. To date, the only functionally characterized systems for metal-citrate transport in this family are those of Bacillus subtilis (21), Streptococcus mutans (20), and, most recently, Enterococcus faecalis (5 (20). This suggests the intriguing possibility, given that S. mutans is considered the major etiological agent of dental cavities, that the organism may use the system to access essential iron, and so the system may play a role in pathogenesis.…”

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confidence: 99%

“…Among the possible functions of these genes was the putative ability to take up citrate in a complexed, metal-bound form. A comprehensive phylogenetic tree of the CitMHS family reported by Blancato et al shows this S. coelicolor CitMHS member on a branch well separated from those systems investigated to date (5). The transporters on S. coelicolor's branch actually share only between 35 and 45% AA sequence homology with those transporters investigated to date, compared to the 60 to 83% AA homology between the B. subtilis, E. faecalis, and S. mutans transporters.…”

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confidence: 99%